The invention relates generally to the laminates which are widely used to make printed circuit boards. In one application, the invention is applied to the composite material used as a substrate for such circuit boards, particularly those made with glass fiber reinforced epoxy resms.
The generally recognized types of laminates are discussed in Printed Circuits Handbook, Coombs ed., Third Edition, McGraw-Hill Book Co., 1988. The lowest cost materials typically use phenolic resin impregnated paper (FR-2) and are used where the cost is more important than the electrical and physical performance. FR-3 is a paper composite which has been impregnated with epoxy resins rather than phenolic resins. CEM-1 is a composite which is more expensive than the FR-2 and FR-3 materials, but which provides improved electrical and physical properties. For CEM-1 an epoxy resin is used to coat paper as in FR-3, but the core is covered with glass fiber reinforced epoxy resins outer layers. FR-4 is made with a fabric of glass fiber yarns impregnated with epoxy resins. The present invention has application to all types of laminates, but in particular to FR-4 laminates.
Inorganic fillers are disclosed in U.S. Pat. No. 5,264,065 to be useful in controlling the coefficient of thermal expansion in the Z-axis of laminates, generally using 30 to 100 parts of filler per hundred of resin. Such fillers have been used in laminates for other and related purposes, such as to limit resin flow and to improve punchability according to the Japanese patent publications (JP 222,950 (1989), JP 199,643 (1982) and 7,044 (1984), JP 97,633 (1989), JP 120,330 (1990)) discussed in the '065 U.S. patent. In U.S. Pat. No. 4,960,634 zinc oxide is disclosed as an additive for improving thermal conductivity. In JP 133,440 (1990) "burned" talc is disclosed to improve dimensional stability of laminates. The talc was heated to 1000.degree. C.-2000.degree. C. to remove the water of crystallization. Talcs heated to below 1000.degree. C. were stated to lack the desired improved dimensional stability.
The present inventors have been concerned with problems arising from the use of epoxy resins having relatively high glass transition temperatures (Tg), particularly about 150.degree. C. or higher. These resins have advantages not available with those having lower Tg values, but they tend to be more brittle and more difficult to work mechanically. They produce epoxy resin dust when laminates are punched, cut, or drilled during fabrication of printed circuit boards. Dust is undesirable for various reasons, but particularly because it affects the precision with which circuit patterns can be made.
The present invention provides a means to significantly reduce the amount of dust produced, with the attendant advantages in manufacturing printed circuit boards. The invention also provides laminates which may be more readily drilled. Typically, laminates are stacked and drilled at the same time for efficiency. If the holes are not drilled cleanly, reworking or rejecting the board may be necessary. Alternatively, the drilling process may be modified, but this may reduce the speed with which the laminates are drilled. An additional advantage of the invention is that the epoxy resin has lower coefficient of thermal expansion (CTE) in the Z direction, which reduces circuit failures due to differential thermal expansion. These advantages are attained by including minor quantities of talc in the epoxy resin. However, it has been found that not all talcs can be used for some cause a substantial reduction in electrical strength.